* Question 1
Needs Grading | | | What is the helium fusion reaction, and why does it require much higher temperatures than hydrogen fusion? | | | | | Selected Answer: | Helium fusion reaction is the transformation of 3 Helium nuclei into one carbon nucleus. It requires much higher temperatures than hydrogen fusion because it has a greater positive charge (two protons), which requires a strong force to deal with electromagnetic repulsion. | Correct Answer: | The helium fusion reaction is the "triple alpha process". In this process three helium-4 nuclei (the alphas) are converted into one carbon-12 nucleus.The triple alpha process requires higher temperatures than proton fusion because the Coulomb barrier is larger. | | | | | * Question 2
Needs Grading | | | What happens to a low-mass star after it exhausts its core helium? | | | | | Selected Answer: | The low-mass star dies after exhausting its core helium. | Correct Answer: | The process of fusing helium creates carbon. Eventually the core will run out of helium, leaving an inert carbon core surrounded by a helium fusing shell (still surrounded by a hydrogen fusing shell). Both shells will contract, creating higher temperatures forcing the star to expand to a double shell-burning giant. The double shell burning will last a few million years or less. The star will never again achieve equilibrium like it had on the main sequence. Instead the star will undergo thermal pulses where the fusion rate increases dramatically every few thousand years.The inert carbon core of a low-mass star will never get hot enough to fuse the carbon. Therefore the end of the star's life is near. | | | | | * Question 3
Needs Grading | | | What will happen to Earth as the Sun changes in the future? | | | | | Selected Answer: | The Earth has a large amount of water vapor and carbon dioxide, both gases. Waters are in oceans and carbon dioxide in rocks. By increasing the solar radiations on Earth, waters will boil, rocks will sublimate life on Earth will be a disaster. Earth will be destroyed. | Correct Answer: | In about 5 billion years, the Sun will use up its hydrogen supply and become a red giant star. The outer edges of the Sun's atmosphere as a red giant will likely swallow poor little Earth.Unfortunately, things will get very uncomfortable before the Sun becomes a red giant. As hydrogen is converted into helium, 4 particles become 1 particle. Therefore the number of particles in the core decreases, causing the core to contract. The contraction in turn causes the core to slowly heat. Eventually (in about two to three billion years) the Sun will become so hot that our oceans will boil away. | | | | | * Question 4
Needs Grading | | | How does the life of a high-mass star differ from that of a low-mass star? | | | | | Selected Answer: | The early life stages of a high mass stars life are similar to the early stages of the sun's life, except they proceed much more rapidly. But the late life stages of high-mass stars are similar to those of low mass stars. As its core hydrogen runs out, a high mass star responds much like a low mass star but much faster. Low mass stars do not do not fuse more than helium, but high mass stars do fuse very heavy elements until they exhaust all fusion sources. | Correct Answer: | A low-mass star reaches the end of its life when it runs out of helium in the core. The star will turn into a double-shell burning giant, become unstable, and eventually shed its layers as a planetary nebula. The very dense leftover core is called a white dwarf.Unlike a low-mass star, a high-mass star is able to fuse elements heavier than helium. The most massive stars are able to fuse silicon into iron. No star, no matter the mass is able to fuse iron. Once the core consists primarily of iron, it will contract followed by a massive explosion called a supernova. Conditions are so extreme in the core that protons fuse with electrons creating a neuron star. If the neutron star is larger than 3 solar masses it will become a black hole. | | | | | * Question 5
Needs Grading | | | Why does nuclear fusion of heavier elements occur in high-mass stars? | | | | | Selected Answer: | The high mass stars’ gravitational forces drive the pressure and temperature that are high enough to allow additional fusion reactions, which take place faster and at a higher rate. | Correct Answer: | High temperatures are required for the fusion of heavier elements. A low-mass star is unable to get hot enough to fuse elements heavier than helium, but a high-mass star can fuse elements up to silicon. | | | | |